This invention relates generally to the discharging of electrochemical cells having lithium-alloy negative electrodes and is particularly directed, but not limited, to protecting a high temperature lithium-alloy/metal sulfide cell and batteries from damage due to over-discharging.
Lithium-alloy/metal sulfide cells are characterized by high storage capacity and power capability per unit weight. The individual cells are typically of the high-temperature type and are coupled in series and/or parallel to form batteries for the storage of electric power.
It has long been recognized that overcharge protection is necessary in recharging of electrochemical cells and batteries of such cells to prevent or minimize damage to those cells that are charged to well beyond their rated capacity. This is a particularly difficult problem in a series-connected battery where cells have been discharged to varying levels and are recharged at varying rates depending on their internal characteristics.
Representative literature addressing these problems are shown in U.S. Pat. No. 4,324,846 to Kaun et al. which utilizes a ternary alloy of iron-aluminum-lithium or nickel-aluminum-lithium or cobalt-aluminum-lithium to provide a specific overcharge capacity to afford a limited overcharge protection. Electric overcharge protection has also been employed such as taught in U.S. Pat. Nos. 4,079,303 to Cox and 4,238,721 to DeLuca et al. These patents disclose electrical systems for charging multi-cell storage batteries in a manner which prevents individual cell overcharging. The former patent removes any cell from the charging cycle which reaches a predetermined charge voltage limit, while the latter equalizes the charge of each individual cell at a selected full charge voltage by shunting current around any cell having a voltage exceeding this selected voltage.
Prior to the inventor's efforts, it was not realized the extent to which over-discharging causes irreversible damage in vulnerable cells of a battery. Without some means of protection in both serially and parallel connected cells, over-discharging may destroy cells within a few cycles. The inventor has found that the overdischarge of lithium-aluminum, negative electrodes can result in aluminum dissolution into the electrolyte and aluminum precipitation into the electrode separator following recharge. This build-up of aluminum is cumulative and can partially short a cell after a few cycles. The defective cell or cells in a battery will progressively become worse, reducing the battery output and resulting in overheating of the battery.
In addition, a deeply discharged negative electrode exhibits a low lithium activity phase that further decreases the effectiveness of a cell. For example alpha lithium-aluminum alloy (about Li.sub.0.1 Al.sub.0.9)and LiSiAl both exhibit reduced open circuit voltage and increased polarization as negative electrode material in comparison with corresponding materials of greater lithium concentrations. Accordingly, prior workers have avoided cell designs requiring electrochemical reaction of these low lithium activity phases.
In view of these problems in prior art electrochemical cells and batteries, many of which problems were not previously recognized, it is an object of the present invention to provide an improved electrochemical cell.
It is another object of the present invention to limit or prevent overdischarge damage in a high temperature electrochemical cell.
Yet another object of the present invention is to provide an electrochemical cell including a lithium alloy, negative electrode with the capability of avoiding aluminum dissolution on discharge of the cell.
A further object of the present invention is to provide an electrochemical cell having overdischarge indication and overdischarge reserve.
It is a further object of the present invention to provide an electrochemical cell with means for overdischarge prevention.
A still further object of the present invention is to provide an electrochemical cell with the combination of overdischarge and overcharge protection.
One other object of the present invention is to provide an overdischarge protection method in a battery of secondary electrochemical cells.
It is yet another object of the present invention to provide a method of monitoring battery output voltage for overdischarge protection.